Statistical data from the Health Ministry of China in 2008 indicated that there were approximately 2.127 million new cases of neoplasm in China every year, of which there were about 1.06 million new cases of malignant neoplasm. Meanwhile, there were about 2.685 million existing patients with neoplasm, of which there were about 1.485 million existing patients with malignant neoplasm. Health Minister CHEN Zhu indicated in the 21st World Cancer Congress that Chinese cancer mortality has increased by 80% in the past 30 years, the annual deaths caused by cancer were 1.8 million, and cancer had become the leading cause of death for Chinese residents. According to the survey “China Health Statistics Yearbook 2012,” the mortality rate of malignant neoplasm is increasing. The top five types of malignant neoplasm are lung cancer, liver cancer, stomach cancer, esophageal cancer, and colorectal cancer respectively, wherein the mortality of lung cancer and liver cancer increased the fastest, and these two cancers ranked as having the highest mortality of malignant neoplasm diseases.
In the past half-century, many achievements have been made in the field of tumor therapy. With thorough studies of tumor genetics and biology, multiple intracellular key signaling pathways associated with tumors have been found. Cancer cells transduce the extracellular signal to intracellular transduction and regulate activities, such as continual self-proliferation and apoptosis, via these intracellular pathways, to maintain malignant phenotypes and, on the other hand, to generate resistance against treatments by regulating specific genes and protein products thereof. Abnormity of the MAPK kinase pathway, which leads to uncontrolled cell proliferation and retardant differentiation, is closely related to tumorigenesis. As a result, the MAPK kinase signaling pathway has become a preferred target for cancer drug development.
Serine/threonine mitogen-activated protein kinases (MAPKs, also called extracellular signal-regulated kinases, ERKs) are activated by a tyrosine kinase receptor (e.g. EGF receptor) and/or a cytokine receptor related with the heterotrimer of G protein. MAPKs can interact with intracellular signals triggered by different second messengers, then phosphorylate and regulate the activity of various enzymes and transcription factors (such as NF-κB, Rsk 90, phospholipase A2, c-Myc, CREB, Ets-1, AP-1 and c-jun, etc.). In the MAPK pathways involved in normal and abnormal cell growth, the Ras/Raf/MEK/ERK kinase pathway is one of the most well-researched and most important pathways. Over ten years ago, scientists found that the protein kinase family ERKs is involved in promoting proliferation. The MEK family, the upstream kinase of ERK, was quickly identified in subsequent studies. Then it was found that Raf can activate MEKs. Raf is upstream of Ras, which belongs to the G protein family and binds to activated GTP, which can indirectly activate Raf. Ras gene mutation is found in approximately 30% of malignant neoplasm patients, and Ras gene mutation rate is even up to 90% in pancreatic cancer. B-Raf mutation rate is 50%-70% in melanoma, 35% in ovarian cancer, 30% in thyroid cancer, and 10% in colon cancer. Likewise, MEKs can be activated by MEK kinase (also known as MEKK) which is independent of Raf.
MEKs, also known as MAP kinase kinases (MAPKK or ERK kinase), are bispecific kinases. MEKs can phosphorylate serine/threonine residues and tyrosine residues of MAPK (p44MAPK(ERK1) and p42MAPK(ERK2)) (phosphorylation sites of ERK1 are T202 and Y204, phosphorylation sites of ERK2 are T183 and Y185). The MEK family includes five genes: MEK1, MEK2, MEK3, MEK4, and MEK5. The N-terminus of MEKs is a negative regulatory region, and the C-terminal catalytic domain has the functions of binding with ERKs and activating ERKs. Tests have found that the knockout of regulatory regions of MEK1 would lead to intrinsic activity inhibition of MEK1 and ERK.
MEK1, with a molecular weight of about 44 kDa and 393 amino acids in total, is mainly expressed in adult tissues, especially in brain tissue. A trace of MEK1 expression can also be detected during embryonic development. The activity of MEK1 is triggered by S218 and S222 phosphorylation. Studies found that in NIH3T3 cells, the activity of MEK1 is increased when the two residues are phosphorylated into aspartic acid or glutamic acid, and colony formation is increased as well. The intrinsic activity of MEK1 promotes cell aging and expression of p53 and p16INK4a in primary cell culture. However, the role of MEK1 is the opposite in immortalized cells and p16INK4a or p53-deficient cells. MEK2, with a molecular weight of about 45 kDa, has 79% sequence similarity with MEK1, and its activity is triggered by S226 and S222 phosphorylation. The phosphorylation catalytic activity of MEK1 and MEK2 are different for disparate MAPK isoforms, ERK1 and ERK2. MEK3, MEK4 and MEK5 do not play a role by acting on ERKs.
Currently there are many compounds for specifically inhibiting Raf and MEK via the MAPK signaling pathway in clinical trials and the marketing stage. Whereas sorafenib (Bay 43-9006), marketed in 2006, is a non-specific serine/threonine and tyrosine kinase inhibitor that targets Raf, MEK, VEGFR2/3, Flt-3, PDGFR, c-Kit etc., B-Raf specific inhibitors, such as dabrafenib (GSK2118436) and vemurafenib (PLX4032), showed good clinical results, but the duration is not long enough. Meanwhile, clinical studies indicated that the symptoms of most patients who received PLX4032 effective treatment recurred, and it was suggested that long-term treatment with B-Raf inhibitors may cause acquired drug resistance and make patients insensitive to B-Raf inhibitors. In order to overcome the resistance of patients, MEK inhibitors are often combined with B-Raf inhibitors in clinical therapeutics. The specific MEK1/2 inhibitor Trametinib (GSK-1120212), developed by GlaxoSmithKline (GSK), has now entered the pre-registration stage. Other MEK1/2 inhibitors, such as Selumetinib (AZD-6422), Pimasertib hydrochloride (AS-703026), and TAK-733 etc. have entered the clinical trial stage. However, no interaction data between these MEK inhibitors and ERK1 or ERK2 has been disclosed.
A series of patent applications of disclosing MEK inhibitors have been published, including WO2007096259, WO2010003022 and WO2012162293 etc.
In order to achieve better oncotherapy purposes, and to better meet the market demands, we hope to develop a new generation of MAPKs signaling pathway inhibitors, especially MEK inhibitors, with high efficiency and low toxicity. The present disclosure provides novel structural MEK inhibitors, and it is found that the compounds having such structures have low CYP450 inhibition, good activity, and exhibit excellent anti-proliferation activity of cancer cells.